I'll start right off by saying that at the end of the day it's your account, your rules, your decision, etc. This is just a suggestion I'd like to offer to counter a very commonly parroted statement.

The big 3 FF mobile games are FFRK, DFFOO, and FFBE. I don't really know how FFBE operates, so I'll refrain from commenting or speculating, however I do know that a lot of DFFOO players play or have played one or both of the other two games. In FFRK this was also a commonly parroted statement, but it made a lot of sense there. Half-draws/discount draws were initially exceptionally rare, coming only for fests, than maybe bi-monthly or monthly. Discount draws were at first rare enough that even if they didn't hold prizes we really wanted (though often they did) it was still very beneficial to increase our usable amount of characters. Characters generally only needed 1-2 current-tier weapons to be applicable, sometimes even getting away with very old weapons. When discount banners started being released every couple of weeks, or multiple times within a single festival, players started getting skeptical and hesitant with their Mythril because discount draws were starting to take up a majority of our currency.

DFFOO is very different. I believe, based on the guy who hoarded the majority of his gems, that we have obtained on average ~40,000 gems a month (whoever has a more accurate number please comment and I will edit). Spending 2,500 gems a month out of that certainly sounds cheap, but we don't get half-off draws every month. We get at minimum one a week. Our weekly draws are almost always half-off, LCs up until they include new EXs are half-off, and some special events are also half-off. There are probably 7,500~12,500 gems worth of half-off banners in a month on average. This is a significant chunk of gems you're giving up every week if you're just blindly pulling on half-off banners hoping to slightly increase your amount of usable characters. Maybe that doesn't sound like a lot to you, but imagine giving up 120,000 gems in a year just due to half-off banners. This is obviously an extreme because there's a high chance that several or a lot of those half-off banners have something you like or want.

"But it's always worth the powerstones". No, it isn't. It takes 4 power stones to increase a single LB-tier, and 12 stones to bring a weapon from 0lb to MLB (mainly EXs now, but sometimes 35 CPs too). However, 1 dupe = 4 power stones. This game does not actively promote getting a weapon for everybody. Most characters suck or are missing a significant chunk of power with only 1 weapon, with few exceptions. Most EX clears or final stage LC hard clears, regardless of the team composition, will almost always have characters relying on all of their weapons, especially when mission requirements involve carrying dead weight (obviously assuming the dead weight has no gear). Truthfully, you want all weapons for a character, and you want to get some use out of them for a month .

Absurd luck aside you really want a decent stash if you're interested in a banner. 20k on the low-side if you're not caring about EXs, and a vast amount more if you do. Maybe you don't get the character you want but you will most likely outfit A character. More tickets make up for less gems, obviously. The thing, pulling like this will often give you the majority of the powerstones you'll need for that banner. Supports such as Rem, Yuna, or Lilisette generally do very well with just their 15cp passive and a 0-1lb 35cp, which is good. You can save your powerstones if need be. DPS characters really want their full power awakened, and will most likely be the main consumers of your powerstones. This is the only way DPS characters still relevant past a few months.

Example: Cloud, even as a 60-cap character, for example is pretty hot garbage without his EX. Low longevity, low damage, and his launch kinda sucks since he can't dump his BRV with it. His EX gives him an ability capable of crazy strong damage, and gives him a massive BRV regen to offet his low damage. But wait, you want the EX to be 2LB to make the Mako buff actually useful, and you want the MLB so that you can cast his damaging ability more than a few times a fight. However, this pretty much demands that you have his 15cp and 35cp passives as well. I can almost guarantee you were better off just pulling 1-2 times on that Campaign banner than doing 2-4 half-off banner pulls and converting your tears to powerstones.

If you focus on a select few characters, their dupes will usually suffice, with the "black sheep" / unwanted pulls of whatever banner you're pulling on usually sufficing for power stones. Two prime examples are the current Snow and Gunner banners.

This Snow banner is almost strictly for favoritists only. Most of us know the characters have very little current or foresee-able value. Yes Firion gets buffs, and is honestly not a bad character, but he is currently nearing the end of his rope until buffs / EX (in which case you're better off pulling there and a probably better banner). I don't want to make assumptions, but most of the people who carried a dead-weight Snow for the EX probably used Firion only if they didn't have Cloud/Noctis fully geared or they really liked hm. Penelo is...penelo. She's like a Yuffie but with her 35CP, and that says a lot. While 60-cap Snow will be miles ahead of current Galuf or Celes, he is definitely second to WoL who is also getting an EX soon on a very meta-defining banner. Yet I see countless posts of people who say things like "Yea they're all bad but free powerstones lol" or "Meh maybe I'll get a Firion 35cp that'll be something I suppose" ~The guy who has all 3 EXs so far and will probably never use him~.

The gunner banner is also a pretty big trap. Sazh is dead. Balthier is way too weak even MLB'ed, King is not competing with the 60-cap DPS, and Vincent, while his niche' has saved players time and time again, will probably see very little end-game use until his 60-cap (which is admittedly a very good investment), which again, just pull there with presumably better stuff.

It's just a suggestion, but I strongly urge you to research future character banners and make considerations before blindly throwing gems at half-off banners just for the sake of being half-off. You're wasting more gems than you realize, and probably spending more powerstones than you're making. Obviously it's if a half-off banner with items you want, embrace it.